Cancer rates on Treasure Island not higher than normal, study finds

An investigation by the California Cancer Prevention Institute has determined that there is no evidence of higher-than-expected cancer rates on Treasure Island — a former Navy base that is being readied for redevelopment and has been home to hundreds of San Francisco residents over the past 15 years while cleanup activities are underway.

But there appear to be some caveats to those findings — including the small size of the island’s population and a lack of information about how long people have lived there.

The institute, at the request of an unnamed resident, examined cancer rates between 2002 and 2011 among island residents, using the Greater Bay Area Cancer Registry, which collects information from doctors, hospitals and others required by law to report cancer incidences. Epidemiologist Meg McKinley and research scientist Christina Clarke found that there were 48 total cancer diagnoses over those years.

The scientists then looked at “reference rates” for all cancers in the surrounding counties. Their findings indicated “that cancer rates among residents of Treasure Island are not statistically different than other similar communities in the Greater Bay Area.”

But the memo also noted that the small size of Treasure Island’s population “makes any sort of meaningful site-specific statistical analysis infeasible,” and that the results can only be used for what they called “educational purposes.” The state’s cancer registry data doesn’t include information on how long someone lived in a certain area, the authors wrote, “which means that if there is some environmental concern, these data cannot be used to link any perceived increase in incidence to potential exposures.”

Additionally, the memo states that the population of the island — currently about 2,000 — has fluctuated greatly over the years, and that “in such a mobile community, individuals with cancer may not have lived in the area long enough for their cancers to have a common cause.”

“A statistically significant cancer cluster involves a greater than expected number of cases given the size, age and gender distribution, and local cancer incidence of the population,” the memo concludes. “Though investigations of most clusters determine there are not more cases than expected by chance, we consider it good public health practice to respond to community concerns about every cancer cluster, perceived or real. Our investigation revealed that there is no evidence of significantly elevated incidence rates of all cancers among the residents of Treasure Island.”